Inside is both a Poké Mart and Center. At the Mart get around 50 Full Restores and 50 Reveive. If you have extra Pokédollars buy more Full Restores.
Talk to the guys blocking the entrance and they will confirm that you have all the Badges.
Your Pokemon Must be atleast LVL 45 and have the following move types: Fighting, Electric, Fire, Dark, Ice, Flying. If you need more Xp spend all your Money and just keep challenging the Elite Four Without using Items. Once you ready then fight with everything you have.
First up is sidney who uses Dark types. Fighting and the odd Fire and Electric moves will beat him.
I use Focus Energy to have a better chance at a crit.
Next Double Kick which almost beats him.
Mightyena will use Sand-Attack. I he used it a lot on your Pokémon just switch out because you will most likely miss every move.
All of the Elite Four will use Full Restore a lot when their Pokémon is almost beat.
Just use your Fighting move.
For Shiftry switch to your fire type to burn him.
For Absol use your fighting move.
Crawdaunt use your Electric type.
For Cacturne your Fire type.
Phoebe the Ghost type is next. Before the fight starts fully revive/heal all your Pokémon.
First is Duskclops who will use Protect a lot.
I use this predictability to set up Spikes which will damage all Pokémon switching in.
I then use Faint Attack my Dark move to attack.
If Duskclops uses Curse switch out if possible, because you will get hit by damage every turn if not. You can switch back in right after if you want.
Next is her second Duskclops. He knows Ice Beam so be careful.
He also has a Sitrus Berry.
When possible revive or heal Pokémon you need for the battle ahead of time ir it might be too late.
Use Dark Moves and Dusclops will be beat.
Next is Banette.
Banette knows Thunderbolt so be careful.
Use your strongest move if your Pokémon is weak to Banettes or any of her Pokémon's moves. Except Normal or Fighting moves. Don't use those.
She has another Banette if your wondering. which is weaker than the other.
Finally is Sableye which is Dark Ghost so Dark moves aren't Super Effective.
Heal and get ready for the next fight.
Next is Glacia the Ice trainer. Have your Electric type first.
For Glalie use your Fire Type.
I used stat boosting moves, Calm Mind, on my Gardevoir and I still needed him for a few more electric hits so i left him in.
She has a second Glalie for your Fire type.
For Walrein use your Electric move.
Walrein is holding a Sitrus Berry.
As for his second Sealeo use an Electric attack.
Now heal up your Pokémon and get your Ice type handy.
If you had an easy time of it so far Drake the Dragon trainer is where things get a little harder.
Use your Ice move to take down Shelgon.
Altaria will use Dragon Dance which will make it a huge threat.
Use your Strongest Ice move to take it down in one hit.
For Kindra take out your Electric Move.
Her Seadra is extremely resilent however so if you have a stat boosting move I would use it now.
He actually switched in Flygon that is resilent to Electric here before I took down his Seadra.
Use your strongest Ice move on Flygon.
Seadra is resiliant to Ice so switch in your Electric here.
If he switched out Seadra like he did with me just use the same strategy as eralier to defeat him.
Salamence is his strongest Pokémon. Make sure your Ice type is fully healed before sending him in.
Once you do use your strongest Ice move.
Make sure you heal before you enter the next room. It has an event so you won't have the chance to heal your Pokémon once you enter.
Wallace is actually the Champion using a team of Water types.
Have your Electric type out to fight Wallace.
Use your strongest Electric move on Wailord because it has a lot of HP. My Gardevoirs PP became empty after dealing with Drake's Seadra.
Be careful of Tentacruels Poison
If you have a Psychic move that will beat him.
Ludicolo can be beat with a strong Flying move.
Whiscash can't be beat by Electric moves. If you have a Grass move use that.
Since Earthquake can miss I stalled Whiscash by constantly healing with Full Restore and then useing Calm Mind after the turn he missed.
After almost max stat boost it OHKO him.
Gyarados is next and can be beat with an Electric move
Gyarados knows Hyper Beam and Earthquake too, I stalled him out just like Whiscash.
Milotic is next.
Milotic will use the move Recover to heal most of his health and use Toxic to damage you. His other moves like surf and Ice Beam is also reasonably strong.
If you can use two strong Electric moves you can beat Milotic.
Be careful though because his Recover can stall the battle out for sometime.
We stalled so Long against each other that both my Gardevoir and his Milotic fainted on the same turn.
After you win your Rival show up to give you advice on how to win.
Now just Record your team in the Hall of Fame and your done.